I am wondering if the following plants can be divided or propagated successfully: Heuchera, Donkey Tail Spurge (Euphorbia), Corsican Hellebore, and Helianthemum.

Answer:

I consulted The American Horticultural Society Plant Propagation book,
edited by Alan Toogood (DK Publishing, 1999), and it says the following:

Heuchera: by division or by seed in spring. Since cultivars may not
come true from seed, I would recommend dividing your plants. Once spring
growth has begun, lift the plant from the ground and remove small
sections from around the edge (look for good roots, and 2-3 shoots).

Euphorbia myrsinites: (Just a note: based on the USDA information that
this plant is invasive in Oregon and banned in Colorado, I would think twice before propagating it. This species does a fine job of propagating itself, apparently. In general, the genus Euphorbia can be propagated by division in early
spring, or from spring to summer, by seeds in fall or spring, and by cuttings in summer or fall, but if you were to propagate by cuttings, you would need to protect your skin from the sap.

Helleborus argutifolius can be propagated by division after flowering,
or by seeds in summer. Test seed capsules for readiness by gently
squeezing. If the seed capsule splits to reveal dark seeds, it is ready
for harvest. Wear gloves! H. argutifolius (Corsican hellebore) often
self-seeds. Check around the base of the plant in spring. When each
seedling has at least one true leaf, gently lift and transplant to moist,
fertile soil in light shade.

Helianthemum can be started from greenwood cuttings rooted in summer
and fall, and by seeds sown in spring in a frost-free location.

If you would like further information on the relative ease or difficulty
of each of these methods for each of these plants, I recommend coming to
the Miller Library and looking at our books and other resources on
propagation. Here is a link to a booklist.

One of my hellebores did not flower this year. I think the spot became too sunny with removal of a bush. When can I transplant it?

Answer:

Hellebores should not have a problem with sun. They will do fine with a certain amount of shade in the summer, but according to C. Colston Burrell and Judith Knott Tyler's book, Hellebores: A Comprehensive Guide (Timber Press, 2006), "the more sun hellebores receive, especially in spring while the foliage is expanding, the fuller the plants grow and the more prolifically they bloom. Light to partial shade is best for most species and hybrids. The stemmed species such as Corsican hellebore are likely to flop in shade, and they tolerate full sun." The authors also say that it takes 2 to 3 years for plants to bloom at full capacity, so if these are new Hellebores, perhaps they are still getting settled. After 2-3 years, the number of flowering stems should increase.

Have you removed last year's leafy growth? Perhaps if you do this, the plants will invest their energy in the flower stalks. The Burrell and Tyler book says that the winter foliage can cause problems if it becomes entangled with emerging flower scapes. Winter foliage can also attract aphids, which will drain the plant's energy as well. Be careful when removing the old leaves, as the sap can cause skin irritation.

If you wish to move the plants, I would suggest waiting until summer or fall when they are dormant. Moving them might mean you won't get flowers for a while, until the plants settle into new surroundings.

When transplanting, Burrell and Taylor indicate that "Small plants that are not root-bound recover from transplanting fairly rapidly.
Once planted, sparse to moderate blooming occurs the following season. It takes
two to three years for plants to reach full steam." p. 162.

Also, be sure that if you move it you replant it at the same depth it was growing at before, since deep planting can prevent flowering:

"Hellebores buried with their crowns in
the soil exhibit inferor flowering, if they bloom at all, though they continue
to produce foliage. The crowns produce short vegetative stems that raise the
leaf buds up to the soil surface, but in our experience, when buried alive
seldom flower." p.167

It is always hard to know the precise reason a plant fails to produce flowers, as there are many possible causes. I recommend removing the winter leaves, and waiting to see if the flowers return next year. I would not move the plants just yet, unless the site has become scorchingly hot.

I am noticing that the flower bud shoots for my hellebores (Helleborus x hybridus), are starting to push up above the soil surface. There is still a substantial stand of foliage in good condition.

My question is about pruning. I know I'll need to prune about half the leaves away (I understand that the cut should be made at the base) to give the flowers more visibility. Does it harm the plant to prune it during this cold snap? Does it harm the plant to cut ALL the old leaves off in December as the bud stalks begin to appear?

I would appreciate any guidance you can give me, such as when and how extensively to prune them.

Answer:

According to Hellebores: a Comprehensive Guide by C. Colston Burrell and Judith Knott Tyler, "all the hybrids maintain their foliage (...) throughout all or part of the winter (...) In any case, as the flower buds begin to stir in the center of the rosettes, it's best to remove all the foliage to make way for the flowers. Nothing spoils the garden display like a tangle of flowers wrestling with winter-burned leaves. The juice is caustic and sometimes causes a rash, so take care when removing the old leaves."

In The Gardener's Guide to Growing Hellebores, Graham Rice and Elizabeth Strangman advise a more time-intensive method:
"The best approach is to cut off some leaves during the autumn and early winter when the garden is put to bed, concentrating on removing dead leaves and any showing signs of blackening (...) By Christmas time they should be thinned out sufficiently to leave a good circle. However, as our winters become windier it may be wise to remove them entirely at this stage. (...)Thin the leaves further as the flower stems emerge, then just before they are in full flower remove all the old leaves. (...) To compensate for the removal of the last of the leaves the plants deserve a good mulch." They go on to suggest compost or a mulch of leaves for this purpose. The cold snap is unlikely to harm even recently pruned hellebores, as they seem to thrive in the cold.

Garden Tool: Once you've spent a pretty penny on a new hellebore you will want to protect your investment by learning exactly what they need to thrive. Read The Gardener's Guide to Growing Hellebores by Graham Rice and Elizabeth Strangman (Timber Press, 1993) for details on cultivation, propagation and essays on hellebore species and hybrids. Color pictures will whet your appetite for more of these delightful winter bloomers.